Interchannel noise suppressor circuits



April 13, 1954 B. s. vlLKoMERsoN. 2,675,468

INTERCHANNEL NOISE suPPREssoR CIRCUITS Original Filed Nov. 24, 1948 2Sheets-Sheet l ATTORNEY April 13, 1954 B. s. VILKOMERSON INTERCHANNELNOISE SUPPRESSOR CIRCUITS 2 Sheecs-Sheetl 2 Original Filed Nov. 24, 1948A| S a il @i j? '76 E7] INVENTOR BENTJ'Hm'IN Ei .YILKUmERs [1N ATTORN EYPatented Apr. 13, 1954 INTERCHANNEL NOISE 'SUPPRESSOR CIRCUITSBenjamins. 'Vilkomersom Camden, N. J., assignor to Radio Corporation ofAmerica, 'acorporaton of Delaware Original application November 24,1948,1Seria1 No. 61,864, now Patent No. 2,630,527, dated March 3, 1953.Divided and this application August li, 1952, Serial No. 3il2,898

2 .(llaims.

l This invention relates 4to `muting circuits for broadcast receiversand particularly -to combined interchannel noise suppressor :and .tuningindcator circuits for amplitudemodulated CAM) orfrequency-modulated (FM)`carrier Wave :receivers.

The `present invention is a, division :of cepending application -iiledon Noven-iber .24, 1948, Serial No. l61,864, which `"matured into U. S.Patent 2,630,527 on `March 3, i953, and assigned Ito the assignee offthe present application. 'flhe invention in accordance with the parentapplication provides means in -a radio broadcast receiver of thetypeadapted to receive .frequency-modulated carrier waves whereby a:normally-conductive muting circuit is adaptedto'be:de-energized uponreception of a AcarrierWaJ/e. Theaudio-channel of the FM receiver isnormally non-.conductive and is adapted to beenergized .upon #receptionof the carrier Wave. The fmuting circuit also controls a circuit whichindicates :the :amount and sense of mistuning of the receiver circuit.In accordance with vthe present invention similar circuits are ladaptedltoloe used .inra radio broadcast receiver of the type .whichreceivesampli tude-modulatedf.carrier'waves.

It has long been recognized that 'it'.is Adesiralole to mute the audiochannelof a broadcast reeeiver while the .receiver is tuned rbetween:adjacent channels. Furthermore, vthe receiver should ce mutedwhen:ftheamplitude of thefreceived carrier Wave falls lbelow a:predetermined value where vthe Wave would 'che below `marginal utility.Another requirement .of anoise muting or interchannel noise .suppressioncircuit fis :that the audio channel shouldn'ot be openediby intermittentnoise pulseseven when they have high ypeak amplitudes. 'On the `.otherflrand, .Whenthe audio channel is topened 4vby la, carrier Wave of4suiiicient amplitude 'it Ashould not close again when the :amplituderof:the carrier'wavefdecreases momentarily. A'furt-her ,requirement of aninterchannelncise suppressor `circuit is that .the audio channel shouldbe suddenly opened completely should not remain in a semiconductingkcondition whena carrier Wave of .marginal utility is received. Numerousmuting circuits yhave been devised heretofore but none of :them meetsall the above requirements.

It is accordingly an object of the present `invention to provide novelmuting circuits fora broadcast receiver, whereby `the 4audio channel of'the receiver is normally rinoperative and -is suddenly 'opened Aorrenderedully operative in response `tothereeeption of a carrier Wavehav- 2 ing -a :predetermined Iand adjustable minimum average amplitude,vthe vcircuit `being arranged `in such a manner that even highintermittent'noise pea-ks will ynot open the Vaudio `channel while'amomentary reduction of `the amplitude of ythe received carrier Wave willnot closeva previously v opened audio channel.

Another object oi fthe invention v-is to provide a mutingcircuit of thetype-described 1w'here`the muting amplifier serves the additionalfunction of rendering a normally inoperative Felectronic tuningindicator operative in response'vtothe reception of a carrierwave.

lA further ohjectfof the invention'isto vprovide va :combinedinterchannel xnoise suppressor Aand tuning indicator lcircuit lior an-AM'receiver Where the kmuting vamplifier `of the :noise 4l`suppressorpir- Vcuit will control the tuning yindicator tube :to render itoperative when :a :carrierwvave is received and -Where further means:are provided to indicate vsuallyzthe extent-'and sense of mistuning ofAthe receiver.

A modulated carrier Wave receiver 4in Aaccordance with the presentinvention may include :a --d'etector :and a Anormally inoperativeaudioamplier, the vdetector'having a vcommon cathode with the audioamplifier. vThe load `circuit of the detector :controls `a 'mutingamplifier 'which is normally conducting. Furthermore, :a pre--determined vbias voltage is impressed on the Vcontrolgrid of 'themuting amplier. The cathodes :circuits of the muting amplier and of thecomzbined detector and :audio amplier have .a `.com- :fnron impedance.Consequently, when a v"carrier i .Wave vWhose,positivepeak'value-exceeds the cathode potential is received, thee'negativedirect acurrent voltage developed .across Athe detector load.circuithiasesthe gridfand thus reduces thefspace nurrent owinglthroughthe 'muting .amplien .This will, inturn, reduce the cathode potential,of ethe detector lso that the 'relative peak :radioreqnency 'potential'of 'the signal will increase. Accordingly, Aa vlarger direct-currentvoltage is ,developed across the detector rload .circuit which furtherreduces the space current through the :muting amplier. The -muting-amplier yis thus suddenly cut cil While 'the'audio'amplierchan- 4nellis just as suddenly opened. (The predeterfln'in-ed bias voltage applied"to the routing amplifier control grid determines'theminimum averyagepeak amplitude of `a carrier vWave which YWill i open the audio channel.

The muting amplifier may be vcoupled 'to an electronic ltainingindicatortube'torenderits target bright when a carrierwaveis received.'The 3 tuning indicator tube will indicate the amoun and sense ofmistuning of the receiver. Furthermore, the muting circuit and indicatortube of the invention may be applied to an FM receiver.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, aswell as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawings, in which:

Fig. 1 is a circuit diagram, partly in block form, of an AM receiverembodying the present inven tion;

Fig. 2 is a circuit diagram, partly in block form, of an FM receiverincluding a combined muting and tuning indicator circuit in accordanceWith the present invention;

Fig. 3 is a graph illustrating voltages developed at different points ofthe receiver of Fig. 2;

Figs. 4 to '.7 are plan views of the target of the electronic shadowindicator tube included in the circuit of Fig. 2 which illustrate theappearance of the target when the receiver is mistuned (Figs. 4, 5 and6) or properly tuned (Fig. '7) and Fig. 8 is a circuit diagram of aportion of an -AM receiver embodying a modified muting and tuningindicator circuit in accordance with the invention.

Referring now to the drawings in which like components have beendesignated by the same `reference numerals, and particularly to Fig. 1

there is illustrated an AM receiver comprising antenna I forintercepting an AM wave. The wave may be amplified by one or moreradio-frequency (R.F.) amplifiers 2 and converted to anintermediate-frequency (I.F.) wave by frequency converter 3 including abeat frequency or local oscillator. R.F. amplifier 2 and the localoscillator of frequency converter 3 are tunable to the desired frequencyby variable reactances such as capacitors i and 5. Capacitors 4 and 5are variable in unison by tuning control knob 6 in accordance withconventional practice.

The I.F. wave derived from converter 3 mal7 be amplified by I. F.amplifier 'i having a tuned output circuit 8 magnetically coupled toinput circuit i3 of the detector. The detector includes tube Ii whichpreferably is a combined diodetriode as illustrated. Tube II has acommon cathode l2 for its diode and triode sections.

' Anode I3 forms the diode section with cathode I2 while the triodesection further includes control grid I and anode l5.

A detector load circuit i 5 consists of resistor I'I shunted bycapacitor i 3 and is arranged in series with input circuit l@ andeffectively between 'anode i3 and cathode I2. Actually, one terminal ofload circuit I5 is grounded while cathode I2 is connected to groundthrough self-bias network 2S including resistor ZI shunted by capacitor22. An automatic gain control voltage (AVC voltage) is derived acrossload circuit I3 and may be impressed through resistor 2t and leads' 25,25 on The modulation or audio signal is also developed across detectorload circuit I6. The audio signal without its direct current componentis impressed upon control grid I4 of the triode amplifier section. Forthis purpose the high potential terminal of detector load circuit i3 isconnected to ground through audio coupling capacitor 23 andpotentiometer 3i? which functions as the Volume control. The audiosignal is obtained through variable tap 3i connected to control grid Itthrough audio coupling capacitor 32. Grid leak resistor 33 is connectedbetween control grid it and ground.

In accordance with the present invention there is provided control ormuting amplifier 35 having cathode 36, control grid 37 and anode 38,Cathode 36 is connected to cathode I2 of diode-triode iI, The directcurrent component of the signal voltage as developed across load circuiti3 is impressed on control grid 31 through resistors fit, 4I connectedto the high potential terminal of load circuit I5.

`The control grid 3'! of muting amplifier 35 is also supplied with apredetermined bias voltage. To this end there is provided apotentiometer including resistors t2 and 43 connected in series betweena positive voltage source indicated at +B and a negative voltage sourceof -25 volts as indicated. By means of variable tap i5 on resistor i3 avoltage of predetermined polarity and magnitude may be derived andimpressed through resistors 45 and I on control grid 3l. Actually, theseries combination of resistors 45, ll and I'I forms a voltage dividerhaving one terminal connected to ground, the direct current component ofthe signal developed across load resistor circuit I'I being in serieswith the voltage of potentiometer 43.

Anode 38 of muting amplifier is connected to +B through droppingresistor fil. A double vane indicator tube #i8 of the electronic shadowtype has its target E@ connected to anode 38 of muting tube 35. The twocontrol rods t9, 3l of indicator tube i8 are connected together.Resistor 52 is provided between target 50 and control rods 49, 5I, whileresistor 53 connects the control rods to ground.

Anode I5 of the triode section of tube I! is connected to +B throughload resistor 5 across which the amplified audio signal is developedwhich is coupled through capacitor 55 to audio l amplifier 55 andreproduced by loud speaker 5l'.

The muting circuit of the receiver of Fig. l op crates as follows. Thebias applied to the con trol grid 37 of muting amplifier 35 is adjustedby means of tap ill to such a value that the amplifier- 35 is normallyconducting. Muting amplifier 35 preferably has a high mutual conductanceso that' a large variation of its plate current is caused by a Variationof its grid voltage. Furthermore, audio amplifier i2, I4, I5 has ahigher amplification factor than that of muting amplifier 35. A highamplification factor as distinguished from the mutual conductance meansthat the plate voltage variation due to a variation of the grid voltageis large; hence a relatively small change in grid bias will change thetube from the amplifying to the cut-o condition.

The space current which normally iiows through muting amplier 35 willdevelop a predetermined voltage across bias network 23 which may be ofthe order of 5 volts. Accordingly, audio amplifier i2, Ill, I5 iscompletely biased ofi because its control grid is 5 volts negative withrespect to its cathode and the tube characteristic tion; 'carrier Wave.yor in the presenceof carrierwave Abelvv the predetermined; minimumlamplitude,

gemenos slsuclrzthatzonly about 3; voltssis necessary to cutioffitsplateacurrent;`- Letziioncw be assumed thatan; ampli-tudefmodulatedcarrier; wave is tunediniby rotationotuning.controhknob (if. If the;averager peak. amplitude of;A the.; AM waveimpressed.throughinputcirruit. Iy on; detector.' t2, [3i exceeds 5:volts, detection.Willtakeiplace. If the average peak` .amplitudeofltheireceivedwvave islessthan volts, no'rectication canitakaplacebecauset cathode l2' is 5: voltsI above: ground? while anodel'isinormally, tliatlis,.intneiabsence ofa signal, .substantially atground. potential.

As. soon4 asY they received carrier: Waves. isz` des tected,. ani audiosignal. is; developed acrossideL Thel rectiiierh signalcurrent.will.develop a voltageof negative:v polarity at the highpotentialterminal. of; load circuit |631. and thisv negative voltage'.isi impressed through resistors: 441'.' and` 41, upon.. control; grid311. of muting.y amplifier 35; Consequently, the space current fiowing:throughzmuting. amplifier 3.5; is.I rreruced'. thereby reducing. the'.voltage,` drop acrosssbias network. 2D'. The result is. that?. thepositive; voltage. of. cathode l2: with respect. to

ground is; reduced.. rlhisWilL increasetheeiiiciency ofv rectication.of. diode. i2, I3 thereby causing a larger'negative voltageto.. beeapplied to'. the. control. grid of. muting tube. 35i Whicllf. is thussvery.v rapidly: cut oil'.rv 'Ihis;. in. turn',v will render: the. audio.ampliiiereV |21,-Y Mg. I5 suddenly r conducting because its cathodepotential. apfproachesi groundpotential Which isthe: normal potential:of gridy i4;

Theaudio; channel is'. accordingly instantaneing; a mean; amplitudeWhich; is'.` larger than. a predetermined. minimum. value.. This minimumvalue is adjustable: by means of: tap. 44.; sc:y that carrier Waves oflessrthan marginal.. utility can be. prevented from; openingthefaudiof.4 channel. The audio channel will opensuddenlysd that:i.t iseither.` fully. opened. or` fully` closed: andi no semi-conductingcondition. can. exist..

The time .constant of: resistor 1H- and' capacitor 39Iin vthegr'dxcircuitof control. gridi is: large compared' to a cycle. cti themodulation. signal and maybe ofthe orden oil ,l second;l Consequently,intermittent'noise pulsesfofi high: peak amplitudecannot open theaudiochannell be.- cause they cannot' develop. afn negativey voltage across:vresistor il and capacitor.. 39 which is suicient'.. to cut; off: the.vmutingal amplien. On

vthe other hand, even relatively; Weak continuous carrier Waves Will be'able to open-v the audio channel as longas'their'meanamplitude exceedsthe predetermined minimum value.

The delay voltage. injected through; resistor 2i` on AVC leads ,252 2.5has the; follovfinggfunc- Normally, that is; in'jthe'-l absence: of" ano AVC voltage is'. developed. Accordingly, the resonant. circuits 'onwhichtheAVC voltageA is impressed are damped-due to the small'residualbias of the grids, andthis will reduce. the amplication in .Rg-F.amplier channelandzin'lfh". amplifier channel. l. I-Iovvever,A as. soonas. a carrier Wave is received which hasv an average amplitude above thepredetermined minimum value, an AVC' voltage is developed Which willovercome the delay voltage. Now, the resonant circuits tov which'theLAVC voltage isapp'lied are 4no' longer damped: so'. that the'amplicatiom of the vvave through; thezReF.: and I.F. channels increased;This faction. ofi' thei ANC: circuit its midpoint.

'willi iurthei admin suddenlyeopeningz the muiio channel...

Tuning indicatorftube 48 is normallyi` inoperrative, that is', as. long:as mutinglamplier 3.5: is conducting space current. In thatY case, thevoltage drop: acrossl resistor: 4i" will. lower; the voltage of. target50,.' tog-such .an extent.: that elec;- tronsfare not,v attracted bytheY target: and.: the targetzwill; remain; dark; However, when mut'-ing. amplierf. is .out off,A the voltage applied to target 50.' becomessufiiciently high to attract; the electrons... Controlzrods 49., 5|areconnected to;- gether and.` maintained through. potentiometer5.2,'.53J' at such. a; positive: potentiakthat: they will not cast ashadow on target 50.. time-astation is; tuned'xin, target 50 Willlightup to indicate; thef presence of' a carrier wave.. of sufficientamplitude even if volume control'. tap 31, is,v turned down'. so. thatnoz: soundf` is heard from loud speaker: 511.,

Eig: 2. illustrates an receiverzprovided? with the. muting circuit of.the presentinvention'.. The tuning; indicatorV circuit included in thereceiver ofgFig. 2-hasbeen .disclosed and'A claimed inthe-:co:- pendingapplicatiorrto'B. SLVilkomerson, Serial No.4 24,371, iiledi on April;30.; 194.8, now.' U. S. Patent 2,502,293, issued. on.- March` 28,.19150;. and assigned 'tof the. assignee.- of this` application; An FM?.Wave. mayA be. intercepted.. by; antenna Ir and amplified by Rz-Fr.ampliiier` stage. 2. TheI R'..F. Wave derived from ampliiier 2iszconvert'ed by frequency converter 3' toran I.1';wavewhich is furtheramplified byone':or-more:I..F-. ampli,- ers 1. R.-F. amplifier2;.andzthe-'localoscillator of frequency converter 3 are,I tunable: to@the desired. frequency by variablev reactancessuch as capacitors ilV and5, variable in unison: by tuning control knobcain: accordance withconventionalpractice..

The: last'. I..-F.;. ampl-ieri stage. 1 has. an4 outlpute. circuit 8comprising; coil Sli'tun'ed. by capacitor :61. I'heainductance' of coilemaybe adjusted` by kparamagnetic core;6-2. LEE-.output circuit 3formsthe primary circuitofj a frequency, discriminator network whichincludes secondary circuit t3A comprising con-64; tuned by capacitor65a- P'rimary circuit. 8. and secondary circuit 531 are; coupled', to.eachother by link circuit 6B comprisingY coil.. 6,1- and: capacitor 6B`arranged in; series. Coils (itl.V and 611 are' magnetically coupled asindicated; One terminal'. of coil' 61 is connected to coil Bl'iat-.atap4 on` one sideof The other` terminal of coil. 6]: is coupledthroughcapacitor S8 tor anotherr tap oncoilv G4- Which is: closely 'adjacentbut on1 the other side:- of. thevmidpoint of coil G4. Primary Thus,

means of paramagnetic coreg 1i), coil. 6.4l` may be:electricallybalanced; Paramagnetic; coreh 'I I serves; the. purpose: offadjusting the'- resonant frequencyof secondary circuiti-35'.. The".balance or; coil` 54 may have.` to be:r readiusted. by" core iii. after.thefcircuit` is tuned b.ycore1'|.

The-frequency discriminator 'network including primary circuitir andsecondary circuit" 63.forms partf off a frequency discriminator.lPreferably, the frequency discriminator'is-of the. ratio detector: typevvhichhas been described by4 Stuart Wm... Seeley.l on.v pager 26 oiithevv December: 19.47 issue; of f.: ENE andl Televisioni-A The ratio.detector illustrated inzFig. zicomprisestwmrectifiers;such as vacuumdiodes 12 and 73.. The,- cathodaiaf Thus,v every 4equal magnitude.

rdiode 12 is connected to one terminal of secondary circuit 63 while itsother terminal is con- -nected to the anode of diode 13. stabilizingcapacitor 14 is connected between the anode of diode 12 and the cathodeof diode 13. Stabilizing capacitor 14 is shunted by resistors 15, 1Bhaving their junction point grounded as shown. Stabilizing capacitor 14presents a low impedance lto intermediate-frequency andmodulation-frequency currents. stabilizing capacitor 14 and resistors15, 16 are chosen in such a manner that they have a time constant whichis larger than 'a cycle of the modulation frequency and which may be ofthe order of 0.1 second. Capacitor 11 'bypasses resistor 16 and has alow impedance to intermediate-frequency currents but a high im- 'pedanceto modulation-frequency currents.

The ratio detector of Fig. 2 operates in a conventional manner. Thefrequency discriminator network is balanced at the center frequency,that is, at the center frequency the carrier frequency voltages impresedupon diodes 12 and 13 are of Accordingly, the voltage at the junctionpoint of coil 61 and capacitor 68 will be equal to that of the junctionpoint of resistors 15, 16 which is a ground potential. However, when theinstantaneous frequency of the FM vwave deviates from its centerfrequency, the frequency discriminator network becomes unbalanced andthe voltage impressed upon diode 12 will be unequal to that impressedupon diode Accordingly, the voltage of the junction point of capacitor68 and coil 61 will no longer be zero with respect to ground.

When an FM wave is received, a voltage is developed across stabilizingcapacitor 14 and resistors 15, 16 which varies slowly with changes inamplitude of the received wave. Thus, an AVC voltage may be derived fromlead 18 which is connected to the junction point between resistor andthe anode of diode 12. The AVC voltage may be impressed in aconventional manner through filter resistor 80 upon I. F. amplifier 'Iand R. F. amplifier 2, as indicated, by leads The demodulated or audiosignal may be obtained from lead 82 connected to the junction pointbetween coil 61 and capacitor 68 and fed through de-emphasis network 83and coupling capacitor 84 to audio amplifier 85. De-emphasis network 83comprises series resistor 86 and shunt capacitor 81. It is conventionalpractice to provide a pre-emphasis network in one of the audioamplifiers of an FM transmitter which attenuates audio frequencies below1500 Cycles more than higher frequencies. De-emphasis network 83 isconventionally provided in an FM receiver for the purpose of attenuatingaudio frequencies above 1500 cycles more than the lower audiofrequencies to establish again the proper balance @60 Vof the audiosignal.

The audio signal derived from de-emphasis network 83 and couplingcapacitor 84 is developed across potentiometer 88 connected betweencoupling capacitor 84 and ground. The audio signal may be taken frommovable volume control tap 90 and coupled through coupling capacitor 9|to control grid 92 of audio amplifier 85. Audio amplifier 85 also has acathode 94 and an anode 95 connected to the anode voltage supply +Bthrough anode resistor 96. The amplified audio signal developed acrossanode resistor 96 :is impressed through coupling capacitor 91 on audioamplifier |00 and may be reproduced by 'lloud speaker 0|.

The output signal of the ratio detector is utilized to an indication ofthe amount and sense of mistuning of the receiver. To this end theoutput signal of the ratio detector is filtered by low-pass or audiofilter network |03 comprising series resistor |04 connected to thejunction point between de-emphasis network 83 and audio couplingcapacitor 84, and shunt capacitor |05. The signal derived from filternetwork |03 is the mean voltage or direct-current voltage component ofthe rectified signal, that is of the audio signal. This mean voltagewill be zero only when the receiver is properly tuned, and will beeither positive or negative when the receiver is tuned to a frequencyabove the assigned center frequency of secondary circuit 63 or belowthis assigned frequency, respectively.

This mean voltage is now impressed o-n direct current amplifier |06including cathode |01, control grid |08 and anode ||0. Control grid |08is connected to filter network |03. .Anode ||0 is connected to asuitable positive voltage supply indicated by +B through resistor lead||2 and anode resistor 41. Cathode |01 is also connected to +B throughresistor ||3, lead ||2 and resistor 41, and is connected to groundthrough cathode resistor ||4 which may be adjustable as shown. Resistors||3 and ||4 accordingly funtion as a voltage divider which will keepcathode |01 at a predetermined and adjustable positive voltage. l

The control signal developed across anode resistor is impressed on oneof the control rods or vanes 49 of a double vane indicator tube 48 ofthe electric shadow type which may be a 6AF6 tube. Indicator 48 has agrounded cathode, two control rods 4S, 5| and a fluorescent target 50connected to +B through resistor 41, as shown. Control rod 49 isconnected by lead ||5 to the anode ||0 of amplifier |06. The junctionpoint :between anode resistor 41 and target 50 is connected to controlrod 5| through resistor 52, and control rod 5| is connected to groundthrough resistor 53. Resistors 52 and 53 accordingly form a voltagedivider which will maintain control rod 5| at a predetermined positivevoltage.

Control rod 5| and target 50 of indicator tube 48 are connected to theoutput of muting amplifier |26. The cathode 2| of' muting ampli- -fier|20 is grounded through resistor |22 and bypassed to ground by capacitor|23, while its control grid |24 is connected through resistor |25 tolead 18 which carries the AVC voltage. The cathode |2| of mutingamplifier |520 is tied to the cathode 94 of audio amplifier 85, whilethe anode |26 of muting amplifier |20 is tied to target 50 of indicatortube 48. "3f

The FM receiver of Fig. 2 including the tuning indicator and mutingcircuit off-the invention Aoperates as follows. In the absence oi'acarrier wave no-AVC signal is'developed and accordingyly mutingamplifier |20 will conduct current.

This is due to the fact that its control grid |24 is connected to theanode voltage supply +B through resistor |28, which, in turn, isconnected to ground through resistors |25 and 15. Control grid |24 willaccordingly have a potential which is sufficiently positive to rendermuting amplifier |20 conducting. The resulting voltage drop across anoderesistor 41 will reduce the voltage of target 50 of indicator tube 48 tosuch an extent that the target will remain dark. At the same time, thespace current of muting amplifier |20 will flow through its cathoderesistor |22. This, in turn, will raise the potential of cathode |f2|las fwellias-that-of cathode .9.4 .o'f aud-io iamplier 85 Whichare tiedtogether. Consequently, audio amplier Will be .biasedto or 'beyond cutoff. Target .50 :of the tuning indicator 48 will remaindark tasillustrated in Fig. :4.

When a=carrierwave is received, aniAVCfSignal is developedxby the ratiodetector whichds impressed through fresistor |25 on control .grid |24 ofmuting amplifier |20. This voltage has been illustratedinlilig. 3 bycurve |33 Aandvaitles as a function of the mistuning of the .recever.Accordingly, a negative AVC voltage is impressed on control grid |24which will cut ol mutine amplifier |20. Hence, the space currentofgmuting tube r| 20 will no .longer ovv through cathode resistor V|22so that the potential-.o both .cathodes |2| and .'94 Will'rise. Audioamplieris now in a condition'to 'conduct space current-.and to amplify.the audio .signal impressed thereon. Atithersametime,*.thefcutting offof mutingiamplier |f2.0 Will raisesthe voltage of target 50 `:so that itcannow attract electrons.

Let it novvibe-assurned that tuning controlfknob 6 is mistunedso`.that-the center frequencyviof .the received FM YWave is lower than theassignedrcenter frequency :value Fi of secondary icircuiti. In thatcase, the demodulated signal Whichis impressed AorrcoritroVgrid |08 ofamplifier .|.|0 has `Va 'positive Lmea-n voltage as illustrated '.bycurve |30 of Fig."3. -Curve |30 thus lrepresents the voltageJimpressed-on control grid |08 `as a functionpf the'tuningfofthereceiver. .Since a positive voltage 'is'impressed on faniplier lt' theamplierwillconduct more space current'than normally with "acorrespondingly larger 'voltage drop iacross y"its "anode vresistor il'`The voltage impressed `through?lead "|lon control rod '#0 io'findicator '2&8 lvvlltherefore decrease. Whenithe voltage of contrdl'rod"49 decreases, itwill repel electrons'and'casta shadow on taaget'50. Theshadow z|32 wappearing-.on targe'ti is :illustrated in `Fig. "5 and itsanglegis controlled .py rod 4t i to indicate theamount of' inistuningof't'her'eceiven *Let it `now be assumed that 'tuning control knob 4(i Iismistuned 'so 'that 'the 'center frequency of A'the `FM iva-veais'higher' than theassigned .center frequen'cy'value "ofsecondary circuitiLNow the'm'ean voltage 30 'of the audio "signal'is negative, and thisnegative voltage Will be impressed upon control grid |03 of 'amplifier106. The space current :through larnpliier :|06 is thus re'- duced soythat the voltage limpressed through lead'rf on-'controlrod W9 Willrise. Contrl"'1od 49 will therefore `attract 'electrons and "anoverlapping bright portion 1:3! Whichiisbrighter ythan the remainder ofthe 'target `vvill "appear f'on target 50 vas shown 'in"Fig. v:6.

If `tuning control knob A"6 Yis 4correct-1y tuned, the mean 'voltage1530 of the `audio signal --(-a't `the frequency'r)is'zeroas"illustratedin'Fi'gjB. Accordingly, a voltage of zero magnitudeis vxirnpressed 'upon -coritrol grid |00 vof amplifier `(00.Cathoderesistor l is adjusted 'in'sucha fmanner that `cathode T01 has a'predetermined positive voltage. LT-.Tnder ithese vvconilitions. fthe'space currentof amplifier tube L|056 Ivis of \sucha"-n'iag nitudefth'at the voltage of control rod-"40 -is Ystiflicient "to cause`feither shadow `|2-or overlapping bright *portioni3-il lju'sttodisappear. Target 'T50 accordingly will vbe uniformly `illuminated 'asshown in lFig. 7. LMutingfampli-ler H preferably yhas a 'flow5arnpli'fication 'ffaotor Fsoitliat the brightness rdf target 50:depends :to` a 'pertain eictentionfthestrengthofithelreceivedfcarrierzwave. In 'other mords, nmless ithe,WVO `:voltage i' |533 :i1-

1i()` lustrateddn Fiaexceedsfa'certain value', muting amplifier :|20will.still .conduct-space currentatoaacertain extent. .The'brightness oftarget 5j0..thus.ir1dicates.theistrength of a wave of marginal strengthxwhich is received.

.dit `fvvll be .understood .that the ratio detectory whichhasbeendllustrated by iwaypf example in Fig.;2 :may :be replaced :by :anyconventional frequency discriminatori'. Furthermore, 1 the AVC voltagewhichV is .impressed von -muting amplifier L20may alsobederived inanother conventional manner. Thus, may bedesired .to'derive the AMCWolta-ge through a sharply tuned circuit so thatrandio amplierrl isvonlyrendered conduct-v Amay .connected tto .-1. .amplifier :1 .of Fig.1.

I. rF.icircui-t8 Acomprises coil -|35.inductively coupledtozinputfcircuit.fwfoftthe detector. Thedetectorrcoinprises diode s|2,|3fhaving-a load-.circuit "lr6 .across which .the saudio signal isdeveloped. .The :audio signal .is .thenimpressed upon control. grid A. 4.of fthez audio .amplifier .in the manner explained .inconnectionmithFig. 1.

iMuting-.ainplier-.a lhas its cathode 3B 4tiedto cathode=z|2of-thefaudio amplifier. Both cathodes are connected sto ...ground`through .self-bias net- Work 20. Control gridlof.mutingampler .35

' iebiasedatia.predeterrninedvoltage .derived fromvol-tageadividerdlzfd. AnodeffB-.of muting aznplierfs :andi-targetofndicatorll are .also connected tcgether. .It iwillfaccordingly `beobviousthatatlieaudioamplifler.iscontrolled by muting l amplier 5.-:in.the .manner .explained inconnection with Fig. 1, While the target -50.otindci cator 48 becomes bright as soon as a carrier Wave is received'and dmodulate'd. .Furthermore, the iaudio channel of -th'e circuit ofeFig. '8 l will not open-inlresponse to intermittent noise pulses aspreviously fexpl'a'ined.

However, fthelci-rouit of `Fig. e differs 'ffrom 'that of Fig. 31 lin'that Lcontrolsrodu!) is -not tied fto control rod fof iin'dicator-.tubeA8 .but i'slmade responsive to fthe iameuntand .sense o'f l mistun. ingof the'freceiver. 'fIo-.thisend tcircuiti'. is provided with avfsecondcoil .136 which is y'coupled to .frequencyfdiscriminator.network431 of a Seeley frequenoy'idiscriminator which hasibeen. discloseden'dclaimed in-U..S. .Patent 2,121,103. |131 .iin-v TheA frequencyVl'.diso:ifi-Initiator network cludes coil tunedfby capacitor..|.40.Th'emi'dpoint of co'il W8 yiis Aconnectedthrough capacitor |4| to the-higl-h alfternatingpotential terminal.

of I. F. circuits LT8.

Frequency discriminatori network .|31 Lfunc-r tions "lin AEa. '-Welliknotvn manner. .A i.prim'ary :bor reference yvoltage fis injectedintothe :secondary circuit |38, |40 through apacitor .0:11. :At 'thesame timeaisecondary `voltage `is developed .fat each terminal of'rthecircuit 438,440. Since the phase of fthe itwo 5'secondary voltages withirespect .to .the primary voltage varies with the difference infrequency .of 'the impressed L vWave and the resonantrequency 'ofcircuit |38,

|40, a resultant Waveiis developed atafeach `termi-- nal-.sof @thercircuit `:.vvlii'chlias :ani-amplitude usepresentative of phasedifference, Which, in

turn, is responsive itdthe frequency i'ifference.' The two wavesare'nowrectiieilloy :diodes 1.7442,

l1 |43 having their anodes connected to respective terminals ofsecondary circuits |38, |40. The cathodes of diodes |42, |43 areinterconnected through load resistors |44, |45 which are bypassed bycapacitor |46 having a low impedance to intermediate-frequency currents.The cathode of diode |43 is grounded while the junction point of loadresistor |44,- |45 is connected to the midpoint of coil |38.

The voltage developed across load resistors |44, |45 is integrated orltered by filter network |03 in the manner explained in connection withFig. 2. Accordingly, the voltage impressed on control grid |08 of directcurrent ampliiier |08 is represented by curve |30 of Fig. 3. Anode ofamplier |06 is connected by lead ||5 to control rod 49 as explained inconnection with Fig. 2. Control rod 49 will accordingly cast a shadow(Fig. 5) or a bright overlapping portion (Fig. 6) on target 50 whenthereceiver is mistuned above or below the correct frequency. When thereceiver is properly tuned target 50 will appear as shown in Fig. '1.The interchannel noise suppressor and tuning indicator circuit of Fig. 8will accordingly give the same tuning indication as that of Fig. 2. Thecircuits of Figs. 2 and 8 can accordingly be combined to obtain an AMand FM receiver where identical tuning indications can be observed onthe target 50 of indicator tube 48 whether an AM or an FM wave isreceived. It will be obvious that since control rod 5| of indicator 48is not required in the circuits of Figs. 1, 2 and 8 it may be omitted.

It will be understood that the circuit specifications of theinterchannel noise suppressor and tuning indicator circuit of theinvention may vary according to the design for any particularapplication. The following circuit specifications are included, by wayof example only, for the circuit of Fig. 1:

Diode-triode amplifier 6AV6 type. Muting amplier 35 6AG5 type (connectedas atriode). g ndic or tube 48- 6AF6 type. Resistor 24 1,000,000 ohms.Resistor 26 15,000,000 ohms. Resistor |1 270,000 ohms. Resistor 2| 1,200ohms. Resistor 54 270,000 ohms. Resistor 30 1,500,000 ohms. Resistor 3315,000,000 ohms. Resistor 42 270,000 ohms. Resistor 43 2,000,000 ohms.Resistor 45 10,000,000 ohms. Resistor 40 1,000,000 ohms. Resistor 4|1,000,000 ohms. Resistor 41 24,000 ohms. Resistor 52 560,000 ohms.Resistor 53 390,000 ohms. Capacitor I8 100 micromicrofarads. Capacitor22 5microfarads. Capacitor 28 .01 microfarad. Capacitor 32 .01microfarad. Capacitor 39 .1 microfarad. Capacitor 55 .01 microfarad.

The following are illustrative circuit specifications for the circuit ofFig. 2:

Tuning indicator tube 48'., 6AF6. Audio amplier 85------ 6AV6.

12 Amplifier |96 6Ai6 (connected as a triode).

Resistor 86 15,000 ohms.

Resistor 88 1,500,000 ohms. Resistor 93 15,000,000 ohms. Resistor 96270,000 ohms. Resistor 15 10,000 ohms.

Resistor 10 10,000 ohms.

Resistor |25 220,000 ohms. A Resistor |28 8,200,000 ohms.

Resistor |22 680 ohms.

Resistor 41 24,000 ohms.

Resistor 52 560,000 ohms. Resistor 53 390,000 ohms. Resistor I 041,000,000 ohms. Variable resistor |4 zero to 5,000 ohms. Resistor ||3220,000 ohms. Resistor 1,000,000 ohms. Capacitor 14 microfarads.Capacitor 11 100 micromicrofarads. Capacitor 81 .005 microfarad.Capacitor 84 .01 microfarad. Capacitor 9| .01 microfarad. Capacitor |23microfarads. Capacitor 91 .01 microfarad. Capacitor |05 .1 microfarad.

The circuit constants of the circuit of Fig. 8 may be the same as thoseof corresponding components of the circuits of Figs. 1 and 2 as givenabove.

There has thus been described an interchannel noise suppressor or mutingcircuit which may be used with an AM or FM receiver. The muting circuitfor an AM receiver will suddenly render the audio channel conductingwhen a Wave of predetermined average amplitude is received. The audiochannel will not open on intermittent noise pulses of high amplitude.hand, the audio channel will not close when it has been opened by an AMcarrier wave when the amplitude of the carrier wave momentarilydecreases. The muting amplier of the noise suppressor circuit may servethe additional function of rendering a tuning indicator tube operativeor inoperative. Furthermore, the tuning indicator circuit may include afrequency discriminator and amplifier for obtaining a visual indicationof the sense and magnitude of the mistuning of the receiver.

What is claimed is:

1. A modulated carrier wave receiver comprising a carrier wavetransmission channel, a detector coupled to said channel and having acathode and an anode, a load circuit coupled between said cathode andanode to develop the modulation signal, a modulation signal amplifierhaving a high amplification factor and a common cathode with saiddetector, said modulation signal amplithereon, a frequency discriminatorcoupled toV said channel and having an output circuit for developing asignal whose mean voltage is proportional in polarity and magnitude tothe sense and amount of frequency departure of the frequency of saidwave from the assigned center frequency of said discriminator, a rstcontrol amplifier responsive to said mean voltage, a second controlamplier having a lower amplication factor than that of said signalamplifier, said second control amplier having a further cathode,

said cathodes being connected together, anv impedance element in thecommon cathode circuit of said signal amplier and said second control Onthe other amplifier, means including a source of voltage for impressinga predetermined voltage on said control grid to render said secondcontrol amplifier conducting in the absence of a carrier wave and torender said signal amplifier non-conducting, a connection between saidload circuit and said control grid to cut off suddenly said secondcontrol amplifier and to render said signal amplifier suddenlyconducting when a carrier wave is impressed on said detector having anaverage amplitude exceeding a Value determined by said predeterminedvoltage, and an electronic indicator device coupled to said firstcontrol amplifier for Visually indicating the magnitude and polarity ofsaid mean voltage.

2. An amplitude-modulated carrier wave receiver comprising a carrierwave transmission channel, a detector coupled to said channel and havinga cathode and an anode, a load circuit coupled between said cathode andanode to develop the modulation signal, a modulation signal amplifierhaving a high amplification factor and a common cathode with saiddetector, said modulation signal amplifier having input electrodesincluding said cathode, said load circuit being coupled to said inputelectrodes to impress said modulation signal thereon, a frequencydiscriminator coupled to said channel and having an output circuit fordeveloping a signal whose mean voltage is proportional in polarity andmagnitude to the sense and amount of frequency departure of thefrequency of said wave from the assigned center frequency of saiddiscriminator, a first control amplier responsive to said mean voltage,a second control amplifier having a lower amplification factor than thatof said signal amplifier and a high mutual conductance, said secondcontrol amplifier having a further cathode, said cathodes beingconnected together, an impedance element in the common cathode circuitof said signal amplifier and said second control amplifier, said secondcontrol amplifier having a control grid, means including a source ofvoltage for impressing a predetermined voltage on said control grid torender said second control amplifier conducting in the absence of acarrier wave and to render said signal amplifier non-conducting, aconnection between said load circuit and said control grid to cut offsuddenly said second control amplifier and to render said signalamplifier suddenly conducting when a carrier wave is impressed on saiddetector having an average amplitude exceeding a value determined bysaid predetermined voltage, an electronic indicator device coupled tosaid first control amplifier for visually indicating the magnitude andpolarity of said mean voltage, said device being arranged to beinoperative in the absence of a carrier wave, and a circuit connectionbetween said second control amplifier and said device to render saiddevice operative when said second control amplifier is cut off.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,210,413 Jones Aug. 6, 1940 2,230,578 Title Feb. 4, 19412,341,936 Maynard Feb. 15, 1944 2,451,584 vStone Oct. 19,1948

